literature lends credibility to the calculations. Allen and Bell
reported5a dual reactivity between 6,13-diphenylpentacene
and maleic anhydride as part of a rich study of pentacene-
quinone formation and reactivity. They observed formation
of both a monoanhydride, with cycloaddition reported across
the central 6,13- carbons,5b and a dianhydride, with two
cycloadditions across the 5,14- and 7,12-carbons, respec-
tively, in xylene heated at reflux.
Upon preparing 6,13-diphenylpentacene and 6,13-di(4′-
hydroxymethylphenyl)pentacene6 and separately reacting
each with a 5-fold excess of C60 in CS2 heated at reflux for
24 h, cis-bisfullerene[60] adducts 6 and 7 are formed in 85%
and 75% isolated yields, respectively. When 6 and 7 are
heated at reflux in CHCl3 (bp 62 °C) for an extended time
period, no detectable retro-Diels-Alder reaction results,
thereby confirming the kinetically controlled nature of the
forward reactions run in CS2 heated at reflux (bp 46.2 °C).
1
Figure 1. Aromatic region of H NMR spectrum of 7 showing
Structural Characterization of 6 and 7. A careful
examination of analytical data firmly establishes 6 and 7 as
AA′MM′ multiplets of the pentacene backbone (4H each, 7.43 and
7.56 ppm) along with 4 doublets (2H each) of the slowly rotating
6,13-di(4′-hydroxymethylphenyl) substituents. The large singlet at
7.27 ppm is residual CHCl3 in CDCl3.
1
cis-bisfullerene[60] adducts. H NMR spectra for 6 and 7
each show a single methine singlet at 5.7 ppm which inte-
grates for 4 protons. This shift compares favorably to the
6.1 ppm value for the methine protons in 1.1 Like 1, 6 and
7 each show 1 set of AA′MM′ multiplets that are centered
at 7.45 and 7.60 ppm for 6 and 7.43 and 7.56 ppm for 7.
6) or 7 (for 7) signals derived from the slowly rotating 6,-
13-diphenyl or 6,13-di(4′-hydroxymethylphenyl) substituents,
respectively. Noteworthy, the aromatic methine signals on
6 and 7 are all clustered between 125 and 130 ppm, well
resolved from other 13C NMR signals. A DEPT spectrum of
7 confirms this assignment of aromatic methines. A careful
examination of the region reveals 7 signals for 6 including
5 that are roughly one-half the intensity of the remaining 2
(Figure 2). The 5 weaker signals correspond to the 5 aromatic
1
Unlike 1, 6 and 7 show no XX′ singlets in their H NMR
spectra. The absence of XX′ singlets implicates 6 and 7 as
bisfullerene[60] regioisomers in which fullerene[60] cy-
cloadditions have occurred across the 5,14- and 7,12-carbons
of their respective pentacene backbones. Positive ion elec-
trospray mass spectra of diol 7 show a strong [M‚Na+] signal
at m/z ) 1954,7 confirming formation of a bisfullerene[60]
1
adduct and validating the H NMR interpretation.
Importantly, the aromatic protons of the 6,13-diphenyl and
6,13-di(4′-hydroxymethylphenyl) substituents on 6 and 7 give
1
rise to 5 and 4 unique H NMR signals (2 doublets and 3
triplets for 6; 4 doublets for 7 (Figure 1)), respectively, each
signal integrating for 2 total protons. This result has both
conformational and stereochemical implications. Thus, the
6,13-diphenyl and 6,13-di(4′-hydroxymethylphenyl) substit-
uents on 6 and 7 must be rotating slowly on the NMR time
scale. Moreover, bisfullerene[60] adducts 6 and 7 must
possess a C2V cis rather than a C2h trans structure. C2h
symmetric 8 and 9 would show 3 and 2 unique 6,13-diaryl
1H NMR signals, irrespective of slow phenyl (or 4′-
hydroxymethylphenyl) rotation. Thus, unlike the situation
with C2V 6 and 7, the NMR spectra for C2h 8 and 9 are not
qualitatively influenced by slow 6,13-diaryl rotation.
The 1H NMR evidence for cis stereochemistries in 6 and
7 is fully corroborated by 13C NMR and DEPT spectra. C2V
symmetric 6 and 7 possess 30 fullerenic carbon signals each
in their 13C NMR spectra, including a few with coincidental
overlap in the crowded sp2 region between 135 and 157 ppm.
The additional signals observed in the 42- and 43-line 13C
NMR spectra for 6 and 7 include 6 from the pentacene
backbone (equivalents sets of pentacene carbons: C1′,4′,8′,-
11′; C2′,3′,9′,10′; C4a′,7a′,11a′,14a′; C5′,7′,12′,14′; C5a′,-
6a′,12a′,13a′; C6′,13′) and an additional set of either 6 (for
Figure 2. Aromatic methine region of 13C NMR spectrum of 6.
methines (2 C each) on the slowly rotating 6,13-diphenyl
substituents while the 2 stronger signals correspond to the
aromatic methines on the pentacene backbone (4 C each,
C1′,4′,8′,11′ and C2′,3′,9′,10′). Similarly, the DEPT spectrum
for 7 reveals 6 aromatic methines between 125 and 130 ppm,
4 of roughly one-half the intensity of the remaining 2. C2h
Org. Lett., Vol. 2, No. 25, 2000
3981
Miller, Glen P.
